Device and method for determining the current flowing through a gas discharge lamp

ABSTRACT

The present invention relates to a device for operating a gas discharge lamp, comprising a switch-mode power supply circut for supplying power to the discharge lamp, the switch-mode power supply circuit comprising a half- or full-bridge commutating forward converter with at least a rail line for supplying a rail voltage, a first switching element, a second switching element, and an output node between said switching elements for supplying current to the lamp, and comprising a current-determining circuit for providing a signal representative of the converter current, wherein the current-determining circuit comprises a first current sensing circuit for sensing the current in a first position between the rail and the output node and a second current sensing circuit for sensing the current in a second position between the output node and ground. The invention also relates to a method and an electronic ballast for operating a gas discharge lamp.

The present invention relates to a device and method for determining thecurrent flowing through a gas discharge lamp. The present invention alsorelates to an electronic ballast for operating a gas discharge lamp.

Nowadays power control devices or ballasts are widely used forcontrolling the power supplied to discharge lamps such as fluorescentlamps. Ballasts may be employed, for example, to optimize the preheatingand ignition of the discharge lamp, to maintain a constant power to theelectric discharge lamp for the purpose of maintaining a selected lightintensity, or for the purpose of controlled dimming to a fixed, butadjustable, power level of the discharge lamp.

Modem electronic ballasts comprise a switch-mode power supply (SMPS)connected between the supply voltage (typically the mains) and thedischarge lamp. In a three-stage ballast circuit, the first stage of theswitch-mode power supply comprises a preconditioner, for example adouble rectifier for rectifying the mains (230 V, 50 Hz, 1 phase),combined with an up-converter. The second stage may comprise adown-converter (DC-DC converter), also called a forward or buckconverter, for stabilizing the output current. The third stage of theballast circuit comprises a commutator bridge and ignitor to implement asquare wave current operation of the lamp. In a two-stage ballasttopology, the down-converter and commutator bridge are replaced by ahalf-bridge commutating forward (HBCF) or a full-bridge commutatingforward (FBCF) topology.

The half-bridge commutating forward (HBCF) circuit corresponds to afull-bridge commutating forward (FBCF) circuit wherein part of thebridge is replaced by two (electrolytic) bridge capacitors in series.The ballast in this topology comprises an up-converter in combinationwith a half bridge acting as a double down-converter. This two-stageballast topology for operating a HID lamp is relatively simple andrelatively inexpensive.

The control of the power supplied to the lamp may be based on theoutcome of measurements of various lamp parameters, such as the actualcurrent flowing through the commutating forward coil element (HBCF-coilof FBCF-coil). This converter current may be used as a measure of theactual current flowing through the lamp. Measurement of the HBCF/FBCFcoil current or converter current may be implemented in various ways,each of them having a number of drawbacks.

One of the methods of determining the converter current is to provide acurrent shunt, for example by connecting a sensing resistor in serieswith the HBCF coil. The differential voltage across the sense resistoris measured by means of a differential amplifier. The current flowingthrough the sensing resistor and consequently the actual convertercurrent, i.e. the current flowing through the HBCF coil, can bedetermined from the known resistance value of the sensing resistor.However, one of the disadvantages of this method is that ahigh-specification operational amplifier, that is an operationalamplifier with a large common mode rejection ratio, is needed, resultingin a considerable cost of the ballast. Furthermore, the signals measuredwith the amplifier have relatively small values, because the resistancevalue of the sensing resistor should be as small as possible to minimizethe losses induced by the insertion of the sensing resistor. These smallvalues lead to a poor signal to noise ratio.

A further method of determining the converter current is to use acurrent transformer, for example by connecting the primary windings of acurrent transformer in series with the HBCF coil. The secondary windingsof the transformer will then provide a signal proportional to theconverter current. However, one of the disadvantages of this method isthat not only the high-frequency components of the current signal, butalso the low-frequency components of the current signal are to betransferred. To guarantee the transfer of the low-frequency components,i.e. the low-frequency commutation signal, of the current signal, arelatively bulky transformer is needed.

Furthermore, asymmetrical current operation of the discharge lamp cannotbe detected. During the start phase and/or at the end of life (EOL) ofthe lamp, the lamp behavior may be irregular, causing an asymmetricallamp load of the above-mentioned commutating forward circuit. The lamp,for example, may be conducting in one half period of the duty cycle ofthe switch-mode power supply and may be non-conducting in the other halfperiod. The resulting DC component cannot be determined in theabove-mentioned full-bridge commutating forward circuit. In the abovementioned half-bridge commutating forward circuit, the asymmetrical loadof the half bridge results in a displacement of the midpoint voltage inthe bridge capacitor series circuit, i.e. the voltage that is thevoltage at the junction between the first and second bridge capacitorsis increased or decreased. As a result of this voltage drift, themaximum voltage rating of one of the bridge capacitors may be exceeded,causing damage to the ballast.

It is an object of the invention to provide a device and method fordetermining the converter current and to provide an electronic ballastwherein the above-mentioned drawbacks are obviated.

According to a first aspect of the invention, this object is achieved ina device for determining the current supplied to a discharge lamp by acommutating forward converter, which converter can be connected to arail line for supplying a rail voltage and comprises a first switchingelement, a second switching element, and an output node between saidswitching elements for supplying said current to the discharge lamp, thedevice comprising a first current sensing circuit for sensing thecurrent in a first position between the rail and the output node and asecond current sensing circuit for sensing the current in a secondposition between the output node and ground. By sensing the currents intwo positions, in a position in the upper half of the bridge and in aposition in the lower half of the bridge, only the high-frequencycomponent of the current signal (typically in the range of 30 kHz-250kHz) is to be determined. Sensing of the low-frequency components, suchas the commutation frequency, typically in the range of 50-400 Hz, canbe dispensed with. This allows the use of relatively small currenttransformers, the total volume of which is less than that of the singlecurrent transformer method discussed above. Furthermore, for half-bridgeapplications asymmetrical operation can be detected. This enablescontrol circuitry of the power supply to adapt the duty cycle of theswitching elements so as to correct the midpoint voltage and hence thevoltages across the bridge capacitors to safe values. A DC component canbe detected also for full bridge applications.

Furthermore, the power losses with this measurement method are reducedas compared with the losses arising in the current shunt methoddiscussed above. Also the output signals need no further amplification,which avoids noise or interference problems.

In a preferred embodiment, the first sensing circuit comprises a firstcurrent transformer having a primary winding connected to said firstposition and the second sensing circuit comprises a second currenttransformer having a primary winding connected to said second position,the secondary windings of the first and second current transformersbeing connected in series for providing a combined signal representativeof the converter current.

According to another aspect of the present invention, an electronicballast is provided for operating a gas discharge lamp, comprising:

a switch-mode power supply (SMPS) circuit for supplying power to thedischarge lamp, the switch-mode power supply circuit comprising a half-or full-bridge commutating forward converter with at least a rail linefor supplying a rail voltage, a first switching element, a secondswitching element, and an output node between said switching elementsfor supplying current to the lamp; and

a current-determining circuit for providing a signal representative ofthe converter current;

wherein the current-determining circuit comprises a first currentsensing circuit for sensing the current in a first position between therail and the output node and a second current sensing circuit forsensing the current in a second position between the output node andground.

In a preferred embodiment, the ballast comprises a gate driving circuitconnected to the gates of the first switching element and the secondswitching element and to the current determining circuit for controllingthe switching of the switching elements on the basis of said signalrepresentative of the converter current. The signal representative ofthe converter current is fed back to the control circuitry that controlsthe duty cycle of the switching elements of the switch-mode powersupply. The duty cycle of the switching elements may be adapted by thecontrol circuitry on the basis of this signal.

According to still another aspect of the present invention, a method isprovided of determining the current supplied by a commutating forwardconverter to a gas discharge lamp, the converter comprising at least arail line for supplying a rail voltage, a first switching element, asecond switching element, and an output node between said switchingelements for supplying current to the lamp, the method comprising thesteps of:

sensing the current in the converter in a first position between therail line and the output node and providing a first output signal;

sensing the current in the converter in a second position between theoutput node and ground and providing a second output signal;

adding the first and second output signals so as to provide a thirdoutput signal representative of the converter current.

If the first signal is the current measured in the first position, thesecond signal is the current measured in the second position, and thethird signal is the sum of the current measured in the first positionand the simultaneously measured current in the second position, ameasure is obtained of the current flowing through the HBCF coil. Thiscurrent is a measure of the current flowing through the lamp.

Further advantages, features and details of the present invention willbe elucidated with reference to the annexed drawings, in which:

FIG. 1 shows a schematic circuit diagram of an electronic ballastaccording to a first preferred embodiment of the present invention;

FIG. 2 shows a graph of the current signal of the upper switchingelement, the current signal of the lower switching element, and theconverter current;

FIG. 3 shows a graph of the combined current signals of the upper andlower switching elements and the converter current; and

FIG. 4 is a schematic circuit diagram of an electronic ballast accordingto a second preferred embodiment of the present invention.

FIG. 1 shows a two-stage ballast for a high-intensity discharge lamp(LA). The first stage (I) of the ballast comprises a rectifier 2 forconverting the AC supply voltage (typically a 230 V 50 Hz mains) to a DCsupply voltage and an up-converter or boost converter 3 for boosting theDC supply voltage. FIG. 1 shows a typical topology of a boost converteror up-converter. The boost converter inter alia is composed of aninductor (Lboost), a switching element (T) and a diode (D).

The second stage (II) of the ballast as shown in FIG. 1 comprises ahalf-bridge commutating forward (HBCF) circuit acting as a doubledown-converter. The HBCF circuit comprises a first MOSFET T1, a secondMOSFET T2, a first and a second (internal) body diode D1 and D2, aninductor Lhbcf in series with the lamp, a lamp capacitor Cr connectedparallel to the lamp, and two electrolytic bridge capacitors Cs1 and Cs2connected in series. The half-bridge commutating forward circuit isoperated in the critical discontinuous mode to allow zero-voltageswitching. Each half commutation period (commutation frequency of theorder of 100 Hz), one MOSFET (the first MOSFET T1 or the second MOSFETT2) is operated in combination with the diode (D2 or D1) of the otherMOSFET. Switching of the MOSFETS is accomplished by a duty cycle controlcircuit, as is schematically shown in FIG. 1. This circuit controls theduty cycle of the half-bridge commutating forward circuit. The controlmay be made dependent on the converter current or at least on a signalrepresentative of the converter current, as determined according to theinvention.

The primary windings of a first current transformer CT1 are connectedbetween the rail line and the first MOSFET T1. The current transformermay equally well be connected between the MOSFET T1 and the output node(O) between the two MOSFETS T1 and T2. The primary windings of a secondcurrent transformer CT2 are connected between the second MOSFET T2 andground or between the output node (O) and the second MOSFET T2. Thesecondary windings of the first transformer and second transformer areconnected in series.

FIGS. 2 and 3 show measurements of the current flowing through the coreof first transformer CT1 in the upper part of the half-bridgecommutating forward circuit and the current flowing through the core ofthe second transformer CT2 in the lower part of the half-bridgecommutating forward circuit. FIG. 2 in fact shows three signals. SignalA represents the response in time of the first current transformer CT1belonging to the upper MOSFET (T1), while signal B is the response intime of the current transformer CT2 belonging to the lower MOSFET (T2).Signal C shows the actual converter current as a function of time. Theleft part of the Figure shows a first commutation half period, while theright part of the Figure shows a subsequent half commutation period.

It is apparent from FIG. 2 that only the high-frequency components ofthe currents through the transformer cores are transferred well, whereasthe low-frequency (commutation) frequency components fade out quickly.In FIG. 3, signal D is a signal that is equal to signal A added tosignal B. It becomes clear that the effects of the weak low-frequencyresponse and the mean values are cancelled. The resulting current signalD gives clear zero and peak current information. This information can beused to assess the operation of the converter current and consequentlythe lamp current. The resulting current signal as a consequence may beused to be sure of a more pure AC lamp operation.

Although the low-frequency part of the current signal fades out quickly,the relatively small current transformers still show a small transfer ofthe low-frequency part of the current signal, as can be derived fromsignals A and B in FIG. 2. Especially signal B clearly shows that aftercommutation the zero level slowly approaches the “zero axis”. Thelow-frequency part of the signal has disappeared already after a fewhigh frequency periods. The above-mentioned series connection of thesecondary windings of the current transformers, resulting in signal D inFIG. 3, has the following advantages.

The slight low-frequency transfers of both transformers will cancel eachother out and will not influence the output signal (C). This impliesthat the low-frequency transfer performance of the transformers hasbecome less relevant. To improve the above-mentioned cancellation of thelow-frequency components of signals A and B, the transformers are chosensuch that the low-frequency responses of the two transformers aresubstantially identical.

A further advantage is that the resulting signal D (FIG. 3) is unipolaror rectified. Regardless of the direction (positive or negative) inwhich the commutating forward converter sends the current (cf. FIG. 3,signal E), the maximum value of the current will be positive.Consequently, the peak current detection circuit, which is connected tothe secondary side of the current transformers, needs only to detect thepositive maximum. Also in the case of a zero-crossing, the flank willalways change from a negative flank through zero to a positive flank.

FIG. 4 shows a two-stage ballast for a high-intensity discharge lamp(LA), of which the first stage (I) corresponds to the ballast shown inFIG. 1. The second stage (II) of the ballast shows a full-bridgecommutating forward (FBCF) topology. The FBCF circuit comprises a firstMOSFET T1, a second MOSFET T2, a third MOSFET T3, and a fourth MOSFETT4, first, second, third, and fourth (internal) body diodes D1-D4, alamp inductor Lhbcf in series with the lamp, a lamp capacitor Crconnected parallel to the lamp, and one electrolytic capacitor Csparallel to the second and third MOSFET. The full-bridge commutatingforward circuit is operated in the critical discontinuous mode to allowzero-voltage switching. Similar to the ballast shown in FIG. 1, theprimary windings of a first current transformer CT1 are connectedbetween the rail line and the first MOSFET T1. The primary windings of asecond current transformer CT2 are connected between the second MOSFETT2 and ground or between the output node (O) and the second MOSFET T2.The secondary windings of the first transformer and second transformerare connected in series. The combination of the signal derived from thefirst transformer and the signal derived from the second transformerwill give a signal representative of the converter current.

Since the actual peak current through the MOSFET is measured by means ofthe above-mentioned current transformers during each high-frequencyperiod, the current is always the same, both in the positive and in thenegative cycle part of the low-frequency current. A DC component(amplitude difference between the positive and negative low-frequencycycle part) therefore is not possible.

The present invention is not limited to the preferred embodimentsthereof described above; the rights sought are defined by the followingclaims, within the scope of which many modifications can be envisaged.

1. Electronic ballast for operating a gas discharge lamp, comprising: aswitch-mode power supply (SMPS) circuit for supplying power to thedischarge lamp, said switch-mode power supply circuit comprising a half-or full-bridge commutating forward converter with at least a rail linefor supplying a rail voltage, a first switching element (Q1), a secondswitching element (Q2), and an output node between said switchingelements for supplying current to the lamp; a current-determiningcircuit for providing a signal representative of the converter current;wherein the current-determining circuit comprises a first currentsensing circuit for sensing the current in a first position between therail and the output node and a second current sensing circuit forsensing the current in a second position between the output node andground.
 2. Electronic ballast according to claim 1, wherein the firstsensing circuit comprises a first current transformer having a primarywinding connected to said first position and the second sensing circuitcomprises a second current transformer having a primary windingconnected to said second position, the secondary windings of the firstand second current transformers being connected in series for providinga combined signal representative of the converter current.
 3. Electronicballast according to claim 1 or 2, comprising a gate driving circuitconnected to the gates of the first switching element and the secondswitching element and to the current-determining circuit for controllingthe switching of the switching elements on the basis of said signalrepresentative of the converter current.
 4. Device for determining thecurrent supplied by a commutating forward converter to a discharge lamp,which converter can be connected to a rail line for supplying a railvoltage and comprises a first switching element, a second switchingelement, and an output node between said switching elements forsupplying said current to the discharge lamp, the device comprising afirst current sensing circuit for sensing the current in a firstposition between the rail and the output node and a second currentsensing circuit for sensing the current in a second position between theoutput node and ground.
 5. Device according to claim 4, wherein thefirst sensing circuit comprises a first current transformer having aprimary winding connected to said first position and the second sensecircuit comprises a second current transformer having a primary windingconnected to said second position, the secondary windings of the firstand second current transformers being connected in series for providinga combined signal representative of the converter current.
 6. Method ofdetermining the current supplied by a commutating forward converter to agas discharge lamp, the converter including at least a rail line forsupplying a rail voltage, a first switching element, a second switchingelement, and an output node between said switching elements forsupplying current to the lamp, the method comprising the steps of:sensing the current in the converter in a first position between therail line and the output node and providing a first output signal;sensing the current in the converter in a second position between theoutput node and ground and providing a second output signal; adding thefirst and second output signals so as to provide a third output signalrepresentative of the converter current.
 7. Method according to claim 6,wherein the first signal is the current measured in the first position,the second signal is the current measured in the second position, andthe third signal is the sum of the current measured in the firstposition and the simultaneously measured current in the second position.8. Method according to any of claims 6 or 7, wherein the electronicballast according to any of claims 1-3 and/or the device according toany of claims 4-5 is applied.